Recently, the number of houses and offices equipped with a photovoltaic power generation apparatus (or solar cells) tends to increase. Power generation of the solar cell may be characterized that an increased amount of electric power is generated as the illuminance of sunlight grows higher, while a reduced amount of electric power is generated as the illuminance of sunlight becomes lower due to the rainy weather or other causes and no power is generated during the nighttime when there is no sunlight. For that reason, the power generation pattern thereof is not consistent with the power consumption pattern in houses or other places for many cases. Further, it is often the case that the generated power amount becomes excessively smaller than the consumed power amount. Accordingly, even if a house is equipped with a solar cell, the house is usually connected to an electric power system of a commercial power source so that, when the generated power amount is insufficient, the deficient electric power can be supplied (bought) from the commercial power source, and when surplus electric power is generated, the surplus electric power is reversely supplied (sold) to the commercial power source.
The electric power generated by the solar cell differs in nature from the electric power of the commercial power source. In light of this, the electric power generated by the solar cell is converted, by a power conditioner including an inverter and the like, to an electric power having the same nature as that of the alternating current power of the commercial power source and is used in association with the electric power system of the commercial power source. This makes it possible to use, in combination, the electric power generated by the solar cell in a house or other places and the electric power of the commercial power source. That is, the power conditioner is a device for converting the electric power generated by the solar cells so as to be made consistent with the electric power of the commercial power source in a voltage, a frequency and a phase, and an example thereof is described in Patent document 1.
The power conditioner described in Patent document 1 is configured to boost the voltage of a DC power generated by solar cells by using a step-up chopper, to convert the DC power into an AC power by using an inverter, and to output the AC power. An electric appliance and the electric power system of a commercial power source are connected to the output of the power conditioner. As described above, the power conditioner makes the power generated by the solar cells consistent to be associated with the electric power system of the commercial power source. The power conditioner converts the voltage of the power generated by the solar cells into a voltage higher than the voltage of the electric power system of the commercial power source and outputs the high voltage. Then, the power conditioner supplies the output power to the electric appliance, and reversely supplies (sells) surplus power, which is not consumed by the electric appliance, to the electric power system of the commercial power source.    Patent document 1: Japanese Patent Application Publication No. 2007-97310
When the surplus power is reversely supplied (output) to the commercial power source, the power conditioner increases the voltage of the output power to a voltage higher than the voltage of the electric power system of the commercial power source. However, if high-voltage powers are simultaneously outputted from multiple houses, the voltage of the electric power system of the commercial power source may be considerably increased. Furthermore, the increase in voltage may cause a variation, exceeding an allowable value, in the power of the electric power system of the commercial power source, and reduces the voltage difference between the voltage of the output power of the power conditioner and the voltage of the electric power system of the commercial power source, thereby reducing the amount of power outputted (sold) from the power conditioner to the electric power system of the commercial power source.